Yet Another Puzzle from Enceladus

Enceladus continues to be an unlikely story, a tiny Saturnian moon jetting icy plumes of what seems to be water vapor from the surface of its south pole. Some believe there is even the potential for life here. But how did the ‘hot spot’ that produces this activity wind up precisely at the pole? We’ll know more through future Cassini measurements, but a new study suggests that such a low-density region could cause the moon to roll over, thus moving this material to the polar area while repositioning excess mass at the equator. What’s more, the bizarre Uranian moon Miranda may bear witness to the same phenomenon.

The theory seems to gibe with other aspects of Enceladus including the surface features Cassini has so vividly imaged. The famous ‘tiger stripe’ pattern gives evidence of being made up of fault lines caused by tectonic stress. And the temperature variation at the pole also reinforces the reorientation concept. “The whole area is hotter than the rest of the moon, and the stripes are hotter than the surrounding surface, suggesting that there is a concentration of warm material below the surface,” says Francis Nimmo (UC-Santa Cruz), a co-author of the paper published in the June 1 edition of Nature.

Enceladus roll diagram

Image: This graphic illustrates the interior of Saturn’s moon Enceladus. It shows warm, low-density material rising to the surface from within, in its icy shell (yellow) and/or its rocky core (red). A NASA-funded study says Enceladus might have rolled or rotated itself to place this area of low density at the south pole. Credit: NASA/JPL/Space Science Institute.

To explain the existence of this internal heating, look to tidal forces exerted by Saturn, causing Enceladus to be squeezed and stretched, thus generating heat in the interior. The heated upwelling of material, known as a ‘diapir,’ then rises from the core or the icy shell around the moon toward the surface, leading ultimately to the reorientation scenario the paper discusses. Visual evidence may eventually be provided by Cassini, which can show whether the pattern of impact craters on the moon has itself become reoriented.

The paper is Nimmo and Pappalardo, “Diapir-induced reorientation of Saturn’s moon Enceladus”, Nature 441, pp. 614-616 (1 June 2006).

Science at the Edge of the Solar System

The Interstellar Boundary Explorer is clearly a mission whose time has come. Scheduled for launch in 2008 and recently confirmed for mission implementation, IBEX will provide global maps of the distant interactions where the heliosphere (the ‘bubble’ of space carved out by the solar wind) meets the interstellar medium. All of this at a time when Voyager 1 is thought in some quarters to have already crossed the ‘termination shock,’ that region where the solar wind is slowed as it encounters interstellar gases; some evidence suggests that the spacecraft then moved back into the supersonic solar wind.

Interstellar medium

Image: The Sun’s movement through the local interstellar medium. IBEX should tell us much about the boundary separating the heliosphere from this region. Credit: Southwest Research Institute.

The Voyager findings remain controversial thanks to magnetic field and cosmic ray measurements that suggest different interpretations, but it’s clear that Voyager is at the very edge of the heliosphere. Which makes this a good time to mention the Jet Propulsion Laboratory’s podcast interview with Voyager scientist Ed Stone (California Institute of Technology), which includes information about the latest findings. In the interview, Stone discusses what happens as you approach the edge of the solar System:

Stone: The bubble is created by the supersonic wind, which means as the wind approaches contact with interstellar wind, it must abruptly slow down and turn around and head down the tail, because the heliosphere is shaped like a comet and so where it abruptly slows down is a standing shock wave, a supersonic shock wave just like in front of a supersonic aircraft. Voyager 1 has now crossed that shock for the first time and is in the region where the wind has slowed down and is turning around to head down the tail of the heliosphere.

Narrator: Okay, so the region it’s in now is called?

Stone: That region is called the heliosheath because it’s the region between the shock and the heliopause, which is the surface of the bubble. That is the edge of interstellar space. Beyond that the material we will be in will be material from other stars. Inside the heliosphere, all the material is coming from the sun and that’s where all the planets are, that’s where all the spacecraft have been, that’s where Voyager still is. But eventually Voyager will exit this bubble and be in interstellar space surrounded by material from other stars.

Firming up our understanding of what is happening to the Voyagers, the IBEX mission is designed to create global energetic neutral atom (ENA) images based on hydrogen ENAs generated in the inner heliosheath, thus telling us much about the properties of the solar wind flow in the regions now being crossed by the spacecraft. Another useful area of research: how the solar wind regulates galactic radiation, crucial data for future probes beyond the Solar System.

IBEX won’t itself travel to these regions. Its target is a highly elliptical orbit reaching 150,000 miles from Earth at apogee. A low-cost mission (first proposed by Southwest Research Institute and now part of NASA’s Explorer program), the spacecraft will be launched from Kwajalein on a Pegasus rocket dropped from an airplane. But it should pay off with big science dividends since the interaction between the Solar System and the interstellar medium has never been directly observed. “Everything we think we know about this region is from models, indirect observations and the recent single-point observations from Voyagers 1 and 2 that frankly have created as many questions as answers,” says principal investigator Dr. David J. McComas. Now comes the needed closer look.

Astrobiology Lectures Available Online

Centauri Dreams continues to champion innovative tools that get scientific findings out to a broader audience. On that score, be aware of QCShow, a freely downloadable player that synchronizes PowerPoint and PDF presentation materials with audio. We’ve discussed this software before, when QCShow’s parent company, New Mexico-based AICS Research, made sessions from NASA’s Institute for Advanced Concepts meeting in 2005 available. Now a weekly series of recorded lectures on astrobiology has launched in this format.

Short of attending a conference on astrobiology yourself, it would be hard to top the list of participants here. Planet hunter extraordinaire Geoff Marcy (University of California, Berkeley) leads off with a 52 minute talk entitled “Exoplanets, Yellowstone & the Prospects for Alien Life.” As the discoverer of roughly 70 of the first 100 exoplanets to be found, Marcy’s thoughts on planetary diversity and its implications for life are well worth hearing, but he’s followed up in coming weeks by, among others, names like Greg Laughlin (UC-Santa Cruz), Webster Cash (University of Colorado), David Grinspoon (Southwest Research Institute) and Matt Golombek (Jet Propulsion Laboratory).

The Astrobiology lecture series can be found here, with archives available. QCShow is a fine tool for distributing this kind of presentation — it is a low-bandwidth solution that focuses on what really counts, the slide show delivered by the presenter coordinated with audio of his or her discussion of the material. My own view is that lectures and conference sessions will one day be routinely distributed through downloadable video files (the burgeoning of digital storage makes it all but inevitable, and preferable to bandwidth-hogging streaming techniques), but as we create that infrastructure, QCShow gets the job done now, and is building expertise for the future dissemination of scientific materials.

Two Thoughts for the Weekend

“Advanced societies throughout the galaxy probably are in contact with one another, such contact being one of their chief interests. They have already probed the life histories of the stars and other of nature’s secrets. The only novelty left would be to delve into the experience of others. What are the novels? What are the art histories? What are the anthropological problems of those distant stars? This is the kind of material that these remote philosophers have been chewing over for a long time…” — Philip Morrison (1961)

“Will we be able to understand the science of another civilization?… Our science has concentrated on asking certain questions at the expense of others, although this is so woven into the fabric of our knowledge that we are generally unaware of it. In another world, the basic questions may have been asked differently.” — J. Robert Oppenheimer (1962)

Of ‘Braneworlds’ and Nearby Black Holes

We’re familiar with four dimensions, three spatial and one temporal. But is there a fourth dimension to space? If so, it implies a new way of looking at gravity. So say physicists Lisa Randall (Harvard University) and Raman Sundrum (Johns Hopkins), who have offered a mathematical description of how gravity’s actual effects might differ from those predicted by Einstein’s General Theory of Relativity. That fourth spatial dimension follows from the theory these two have developed called the type II Randall-Sundrum braneworld gravity model. It suggests that the universe is a membrane, or ‘braneworld,’ embedded within a much larger universe.

Centauri Dreams admires robust theorizing but has always hoped to see solid observational clues that would make such hypotheses testable. And it may be that one has now emerged, in the hands of Charles Keeton (Rutgers) and Arlie Petters (Duke University), who used the Randall-Sundrum model to predict certain cosmological effects that could provide answers, effects that may be susceptible to testing via satellites scheduled for launch within the next few years.

For the braneworld model would have notable consequences. The hypothesis predicts that small black holes from the early universe — with a mass similar to that of a small asteroid — would have survived to the present. Such objects would be part of the dark matter that seems to pervade the universe, exerting gravitational force but reflecting or emitting no light. General Relativity says that such primordial black holes would have evaporated away by now, so finding them would make the braneworld hypothesis tenable.

It would also change our view of nearby space. For remarkably, if such black holes do exist, they may be close. Says Keeton: “When we estimated how far braneworld black holes might be from Earth, we were surprised to find that the nearest ones would lie well inside Pluto’s orbit.” And Petters makes an even more mind-boggling statement:

“If braneworld black holes form even 1 percent of the dark matter in our part of the galaxy — a cautious assumption — there should be several thousand braneworld black holes in our solar system.”

So the object is to look for the effects that these braneworld black holes would exert on electromagnetic radiation coming to Earth from other galaxies. Any such radiation would be subject to gravitational lensing if it came near a black hole. One good place to start is with gamma ray bursts, whose path would be impeded by a black hole to produce an interference pattern. The scientists have worked out the bright and dark ‘fringes’ in the interference pattern that would result; these would offer information on the characteristics of the black holes, and by inference just might change our notions of space and time.

As to missions, the Gamma-Ray Large Area Space Telescope (GLAST), scheduled for launch next summer, may be able to measure such interference patterns. So here is a case where an exotic theory may actually be put to an observational test, and much sooner than many of us would have thought possible. The paper, which appeared in the May 24 online edition of Phyical Review D, is Keeton and Petters, “Formalism for testing theories of gravity using lensing by compact objects. III: Braneworld gravity,” available here.